This invention relates to an apparatus for scanning a laser beam.
FIG. 1 is an overall view of a former laser beam scanning apparatus. A laser tube 10 generates a beam 11. The beam 11 passes through a modulator 12 and optical elements 13 thus becoming a read/write scanning beam 14. The beam 14 is scanning by a rotating polygon scanner 15. The polygon scanner 15 has a plurality of reflecting facets and is rotated by a motor 16. The beam 14 reflects on successive ones of the facets of the polygon scanner 15 and is then scanned at a reading or recording station 17.
Concurrently, a laser tube 20 located on the opposite side of the scanner 15 from the laser tube 10 generates a beam 21. The beam 21 passes through optical elements 22 as a scale scanning beam. The beam 21 is reflected on successive ones of facets of the polygon scanner 15 and is then scanned on a scale 23 for generating sampling pulses. The sampling pulses are used as timing pulses for reading or recording of information on the beam 14.
The laser 10, modulator 12 and optical elements 13 are, therefore, located on the opposite side of the polygon scanner 15 from laser tube 20 and optical elements 22. Accordingly, these tubes and optical elements take up a large space and cannot be used in common. The result is a very expensive apparatus.
In addition, the beams 14 and 21 simultaneously reflect on different facets of the polygon scanner 15. The reflection on the different facets may cause scanning non-linearity because of differing conditions of the surfaces of the facets.
If it is desirable to reduce the optical distance between the polygon scanner 15 and the scale 23, the scale 23 will have to be made smaller. It is difficult to manufacture an accurate smaller scale. Dust or stains accumulating on the scale 23 cause irregular reflections of the beam 21 on the scale with the result that correct timing pulses are not obtained.